Heretofore, it has been known that acrylic fiber can be preoxidized (i.e., made flame-resistant) by subjecting it to a preoxidization (flame-resistance) treatment by subjecting it to tension in air at a temperature of from 200.degree. C. to 400.degree. C., and that carbon fiber can be obtained therefrom by carbonizing said preoxidized fiber under tension in an inert atmosphere at 500.degree. C. and up. (These methods are described in, for example, U.S. Pat. No. 4,069,297.)
Preoxidized fiber has found use as a flame-resistant material, a heat-shield material, a packing material, etc., but there has been a continuing demand for further improvements in the quality and performance thereof.
Carbon fiber produced as mentioned above is being used in sporting goods, leisure goods, automobiles, high speed driving device, for example, centrifugal separator, spacecraft, etc. because of its high specific tensile strength (tensile strength/specific gravity) and specific tensile modulus (tensile modulus/specific gravity), and the demand for it is increasing. Recently, there have been some application areas wherein even currently available carbon fibers having a tensile strength of from 300 to 350 kg/mm.sup.2 have not been satisfactory. In such areas, carbon fibers having much higher tensile strength are required.